IRPLCFL8U
Simplified Three Level Dimming CFL Fluorescent
Ballast using the IRS2530D
DIM8
TM
Table of Contents
Page
1. Features ...........................................................................................2
2. Overview ..........................................................................................2
3. Circuit Schematic .............................................................................8
4. Electrical Characteristics ..................................................................9
5. Fault Protection Characteristics .......................................................9
6. Functional Description....................................................................10
7. Fault Conditions .............................................................................15
8. Ballast Design ................................................................................19
9. Bill of Materials...............................................................................22
10. IRPLCFL8U PCB Layout..............................................................23
11. Inductor Specifications .................................................................27
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1. Features
Drives 1 x 32W Spiral CFL Lamp
Input Voltage: 120Vac
High Frequency Operation
Lamp Filament Preheating
Lamp Fault Protection with Auto-Restart
Low AC Line/Brownout Protection
IRS2530D
DIM8
TM
HVIC Ballast Controller
2. Overview
The 3 way dimming system widely adopted in the US with conventional filament lamps
consists of a light bulb that has a modified Edison screw type base which allows 3
connections to be made to a special lamp socket that also has 3 connections.
Standard Edison Screw
Base
3 Way Dimming Edison
Screw Base
Live
Neutral
Live 1 Live 2 Neutral
Figure 2.1:
Three way dimming Edison screw base
The 3 way dimming light bulb has two filaments inside which produce different light
outputs when connected to the AC line. These filaments are connected in series such that
the mid point goes to the line common and the two ends can be connected to the live
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either independently or both together. Thus with an external switch that has four positions,
it is possible to obtain 3 different light levels or to switch off.
3 Way Dimming Switch
3 Way Dimming Light Bulb
0
Live
120V AC Line
Neutral
1
2
30
1
2
3
60W Filament
40W Filament
Figure 2.2:
Three way dimming filament lamp system
Figure 2.2 shows how the live and the neutral connect for 4 different configurations
(position 0, 1, 2, and 3). The flow of current for each position is also shown with colored
arrow; no current flows for position 0 (switch off), red arrow for position 1, blue arrow for
position 2, and magenta arrow for position 3. In position 1, the current will flow through
the 40W filament resistor (the lowest dimming level). In position 2, the current will flow
through the 60W filament resistor (intermediate dimming level). In position 3, current
will flow through both filaments, and the system will be at the maximum dimming level.
Existing Ballast Solution
There are in existence CFL ballast designs that provide three way dimming based on
the same switching arrangement shown above. A common approach is a system
whereby the line voltage is full wave rectified when one live input is connected and a
voltage doubler circuit comes into operation when the other live input is connected or
both are connected together thereby having two DC bus voltages in the ballast during
dim level settings. This type of design also operates at two different frequencies, a low
frequency (typically 40-45kHz) when both live inputs are connected providing a high
lamp current and a higher frequency (for example 70-75kHz) when either of the two
lives is connected alone which will produce a lower lamp current. In this way the
following combinations are achieved:
1. Low DC bus (150V) / high frequency ….. minimum output
2. High DC bus (300V) / high frequency …... medium output
3. High DC bus (300V) / low frequency …… maximum output
This approach has some serious drawbacks:
Firstly, since the ballast must be designed to give 100% light output for the lamp when
the bus voltage is 300V and the frequency is 40kHz, it is not easy to achieve
satisfactory preheat and ignition when the bus voltage is at 150V because of the
limitations in the peak voltage that the output circuit is able to produce from a 150Vpp
half bridge voltage.
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One strategy that has been used is to omit the preheating phase and steer the
oscillator frequency to resonance during ignition using feedback from the output circuit.
This ensures that at switch-on the highest possible ignition voltage will be applied to the
lamp. In this way the lamp will ignite in whichever position the 3 way switch is set.
Such a scheme could reliably ignite the lamp when the DC bus is at 300V, however
without correct preheating the ignition voltage of the lamp and consequently the peak
current in the MOSFET half bridge during ignition will be higher. Also the life of the
lamp is substantially reduced when there is no preheat due to far greater stress
occurring on the cathodes at the point of ignition.
Ignition when the DC bus voltage is at 150V is very difficult. Tests indicated that
sweeping the frequency down through resonance sometimes failed to produce
sufficient ignition voltage leaving the ballast in open circuit running mode. The
conclusion from this is that the ballast needs to oscillate at resonance for an extended
period of time in order for the lamp to ignite at 150V considering that the output inductor
and capacitor have been designed to produce 100% lamp power at 300VDC bus when
the frequency is 40-45kHz.
Many CFL ballast designs do not incorporate a current sense and shutdown function to
protect the circuit in the case of ignition failure and so the ballast would eventually fail if
left switched on due to the high MOSFET switching losses causing thermal destruction.
This would not matter with and integrated ballast / lamp type product when the lamp
has failed.
It has also been observed that hard switching occurs at the MOSFET half bridge when
the DC bus voltage is low in position 1 since when the ballast is running it will be close
to resonance, bearing in mind that the resonant frequency shifts downwards in run
mode. Hard switching is very undesirable because of the high peak currents that occur
when each MOSFET switches on. This has been shown to result in a higher rate of
field failures in ballasts due to MOSFET failure.
The conclusion is that the approach to design described above is unable to provide a
reliable ballast.
The dimming level can also be controlled by simply changing the frequency. By
changing the frequency between 3 defined settings, however, it was found to be
extremely difficult to set a point where the dim level is 50%. The problem with this is
that the lamp current against ballast frequency characteristic of the system exhibits a
very sharp knee such that as the frequency increases the lamp current is gradually
reduced up to a point at which a small increase of frequency will result in a very large
reduction in the lamp current.
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Ballast / Lamp Operating Characteristic
Lamp
Current
Ballast Running
Frequency
Figure 2.3:
Lamp current against ballast frequency
To obtain 50% output, the frequency would have to be very precisely set. This is not
practical since the tolerances of the output inductor, capacitor and oscillator timing
components do not allow this. Even if each ballast was individually adjusted in
production variations in lamp behavior over temperature would mean that under some
conditions the lamp arc would extinguish at this setting leaving the system in
permanent preheat which would burn out the cathodes eventually.
This explains why the 150VDC bus solution has been adopted in some designs as this
allows 50% output to be achieved without this problem. However as discussed in the
previous section this approach is not without some major disadvantages.
IRPLCFL4 Reference Design
It is however necessary in order to create a reliable design to include a closed loop
feedback system that controls the lamp current by adjusting the ballast frequency from
a VCO (voltage controlled oscillator) driven by the output of an error amplifier that
senses the lamp arc current directly and compares it with a reference. This has been
used in the
IRPLCFL4
reference design “A 3 Way Dimming CFL Ballast” and has been
demonstrated to be capable of controlling the lamp output down to approximately 10%
arc current maintaining stability. This also compensates for tolerances in the
components of the circuit or the lamp.
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